An induction heating apparatus applying induction heating and using an inverter has excellent heating response and controllability by providing a temperature sensor or the like in the vicinity of a pan or the like that serves as the load, sensing pan temperatures or the like, and making an adjustment of the heating power and an adjustment of the cooking time in accordance therewith. The induction heating apparatus realizes delicate cooking, and has characteristics such that it hardly pollutes the air in the room since no flame is used, that it is high in thermal efficiency and that it is safe and clean. In recent years, attention has been paid to these characteristics, and demand for the induction heating apparatus has been rapidly growing.
An induction heating apparatus of a first conventional example will be described referring to FIG. 14. The induction heating apparatus of the first conventional example is capable of heating high-permeability (magnetic) objects to be heated, such as iron (or low-permeability and high-resistance objects to be heated, such as 18-8 stainless steel) and low-permeability (non-magnetic) and low-resistance objects to be heated, such as aluminum or copper. FIG. 14 is a block diagram showing the structure of the induction heating apparatus of the first conventional example. In FIG. 14, reference numeral 110 represents an object to be heated (a metal vessel such as a pan or a frying pan), reference numeral 101 represents an induction heating coil that generates a high-frequency magnetic field and heats the object 110 to be heated, reference numeral 127 represents a commercial AC source input, reference numeral 108 represents a rectification and smoothing portion that comprises a bridge 108a and a smoothing capacitor 108b and rectifies the commercial AC source, reference numeral 1402 represents an inverter circuit that converts the power source rectified by the rectification and smoothing portion 108 to high-frequency power and supplies a high-frequency current to the induction heating coil 101, reference numeral 105 represents a microcomputer, reference numeral 1409 represents an operation portion, and reference numeral 125 represents a cabinet. Reference numeral 118 represents a ceramic top plate disposed on the top surface of the cabinet 125, and on which the object 110 to be heated is placed.
The microcomputer 105 has a control portion 104. The operation portion 1409 has a setting input portion 113 and a setting display portion 114. The setting input portion 113 has a plurality of key switches (including a key switch for inputting an instruction to set the output stage that determines the target output of the induction heating apparatus, a key switch for inputting an instruction to turn on the induction heating apparatus, and a key switch for inputting an instruction to turn off the induction heating apparatus).
The setting display portion 114 has a plurality of visible LEDs (light emitting diodes), and displays the setting condition of the induction heating apparatus.
The control portion 104 drives a driving circuit 111 in response to the instruction inputted from the setting input portion 113. The driving circuit 111 controls the output of the inverter circuit 102. The control portion 104 controls the ON/OFF of a relay (not shown) when a low-permeability and low-resistance object to be heated is heated and when a high-permeability object to be heated (or a low-permeability and high-resistance object to be heated) is heated, thereby switching the number of turns of the induction heating coil 101 operated by the inverter circuit 102 and switching the voltage applied to the induction heating coil 101. The number of turns of the induction heating coil 101 is larger and the voltage applied to the induction heating coil 101 is higher when a low-permeability and low-resistance object to be heated is heated than when a high-permeability object to be heated (or a low-permeability and high-resistance object to be heated) is heated. When the object 110 to be heated is a pan made of aluminum, copper or the like that is low in permeability and low in resistance, the voltage applied to the induction heating coil 101 is not less than 1 kV.
A stray capacitance (equivalent capacitance) 1411 is present between the induction heating coil 101 and the object 110 to be heated. When the user touches the object 110 to be heated, current flows from the induction heating coil 101 to ground through the stray capacitance 1411 and the internal resistance (equivalent resistance) 1412 of the user's body. It is dangerous if current of not less than a predetermined level leaks from the high-voltage induction heating coil 101 to the human body.
Japanese Published Patent Application No. H04-75634 describes an induction heating cooker of a second conventional example that prevents current from leaking from the high-voltage induction heating coil 101 to the human body. The induction heating cooker of the second conventional example will be described referring to FIGS. 15 and 16. FIG. 15 is a block diagram showing the structure of the induction heating cooker of the second conventional example. In FIG. 15, the same blocks as those of the first conventional example (FIG. 14) are denoted by the same reference numerals. The induction heating cooker of the second conventional example is different from the first conventional example in that a conductive electrostatic shielding member 1512 and an insulating layer 1513 covering the electrostatic shielding member 1512 are provided on the undersurface of the top plate 118. The electrostatic shielding member 1512 is connected to a low-potential part of the rectification and smoothing portion 108. Except this, the second conventional example is the same as the first conventional example.
FIG. 16 is a view showing the pattern of the electrostatic shielding member 1512 formed on the top plate 118 of the induction heating cooker of the second conventional example. For ease of understanding, FIG. 16 shows the pattern of the electrostatic shielding member 1512 excluding the insulating layer 1513. The electrostatic shielding member 1512 is applied to the undersurface of the top plate 118 and fixed by baking. The electrostatic shielding member 1512 has an annular shape. The connecting wire from the low-potential part of the rectification and smoothing portion 108 is connected to an electrode 1513 of the electrostatic shielding member 1512.
A stray capacitance (equivalent capacitance) 1514 is present between the induction heating coil 101 and the electrostatic shielding member 1512. Current flows from the induction heating coil 101 to ground through the stray capacitance 1514 and an internal resistance (equivalent resistance) 1515 of the electrostatic shielding member 1512. The impedance of the internal resistance (equivalent resistance) 1515 of the conductive electrostatic shielding member 1512 is sufficiently low compared to the impedance of the stray capacitance (equivalent capacitance) 1514 (the frequency of the high-frequency current flowing through the induction heating coil 101 is approximately 20 to 60 kHz). Therefore, the voltage induced in the electrostatic shielding member 1512 is sufficiently low.
A stray capacitance (equivalent capacitance) 1516 is present between the electrostatic shielding member 1512 and the object 110 to be heated. When the user touches the object 110 to be heated, leakage current flows to ground through the stray capacitance (equivalent capacitance) 1516 and the internal resistance (equivalent resistance) 1412 of the user's body by the voltage induced in the electrostatic shielding member 1512 by the induction heating coil 101. Since the voltage induced in the electrostatic shielding member 1512 is sufficiently low, the leakage current that flows to ground through the internal resistance (equivalent resistance) 1412 of the user's body is extremely small.
In other words, the internal resistance (equivalent resistance) 1515 of the electrostatic shielding member 1512, the stray capacitance (equivalent capacitance) 1516 and the internal resistance (equivalent resistance) 1412 of the user's body are connected in parallel between the electrostatic shielding member 1512 and ground. Since the impedance of the internal resistance (equivalent resistance) 1515 of the electrostatic shielding member 1512 is extremely low compared to the impedance of the stray capacitance (equivalent capacitance) 1516 and the internal resistance (equivalent resistance) 1412 of the user's body, most of the leakage current from the induction heating coil 101 flows to ground through the electrostatic shielding member 1512. Current hardly leaks to the user's body.
In the structure of the second conventional example, when the object 110 to be heated is a pan made of aluminum, copper or the like that is low in permeability and low in resistance, the number of turns of the induction heating coil 101 is increased. At this time, the voltage applied to the induction heating coil is not less than 1 kV. In the second conventional example, since the electrostatic shielding member electrically coupled to the low-potential part is present and there is hardly any potential difference between the object 110 to be heated and the electrostatic shielding member 1512 (their potentials are both close to the ground level), no leakage current is inducted in the object 110 to be heated. Therefore, it is safe even if the human body touches the object 110 to be heated.
The Official Gazette of Japanese Examined Patent Publication No. Sho 55-869 discloses an induction heating apparatus in which a fine pattern (top plate cracking sensing circuit) formed by a conductive coating is provided on the underside of the top plate. A DC current is passed through this pattern. Based on the fact that the top plate cracks and the current flowing through this pattern is interrupted, the induction heating coil is stopped.
The Official Gazettes of Japanese Unexamined Patent Publication No. Sho 62-278785 and Japanese Unexamined Patent Publication No. Sho 62-278786 disclose induction heating cookers in which the conductive fine pattern is provided on the top plate. When the induction heating coil is driven, leakage current flows through this pattern. When the leakage current becomes smaller than a reference value proportional to the output of the induction heating coil, the induction heating coil is stopped.
In the second conventional example, as long as the electrostatic shielding member sufficiently performs its function, it is safe even if the human body touches the object to be heated. However, when the electrostatic shielding member does not sufficiently perform its function for some reason, for example, because of deterioration from aging, safety is not always sufficiently ensured (there is a possibility that a leakage current of not less than a predetermined level flows through the human body when the user touches the object to be heated).
The present invention solves the above-mentioned conventional problems, and an object thereof is to provide an induction heating apparatus with high safety in which leakage current is prevented from flowing to the human body and there is no possibility of an electric shock even when the electrostatic shielding member does not sufficiently perform its function.